It is a good and useful add-on with some minor problems (edited after contacted by support):
- the blacklist disappear after every auto update (so I suggest anyone with this problem to turn off the auto update from the add-on page and backup the list before manual update).
- the blocked sites/pages are still opening/loading in the background tabs/ windows and you still can see their preview in toolbar (in Win 7), see their name and hear any background sound/music they have set on autoplay. Only when you click that specific tab/Firefox window it trigger the block and the page become blocked.
- it really need a very basic feature to instant close the blocked sites tabs. There is no point in keeping those tabs open especially when often their number become really annoying.
My Firefox is v62.0.2 (64bit) and this problems could affect only this version.